Various types of process or chemical indicators are used to measure the concentration of constituents in a liquid or gas environment or to determine the effectiveness of a process under certain environmental conditions. The present invention relates to such process indicators which use permeation or wicking of a substance through a material to react or otherwise cause an indication/measurement of physical condition.
Typical examples of such indicators are illustrated in U.S. Pat. Nos. 3,981,683, 3,932,134, and 4,195,055, which are incorporated herein by reference. Flexible packaging of these indicator devices guides the indicating substance to the barrier or wick. A problem associated with these flexible package indicators, is that changes in environmental conditions, such as temperature and pressure, can cause local deformation of the packaging surrounding the permeation barrier or wick material. As a result, wicking or gas permeation may not be through the specially designed wick or permeable barrier as intended, but through an interspace formed between the wick or barrier and the flexible envelope which forms part of the device. When this occurs, the observer will, of course, receive an erroneous reading which is not related to its exposure.
The present invention is directed to preventing erroneous indications of wicking or permeation indicators by providing a structure which insures that the diffusion or wicking action occurs specifically through the wick or permeation material.